The use of in-hole motors in bore hole drilling, especially in drilling for oil and gas but also in mining operations, has been a standard procedure in the art. Such motors are employed to rotate drills for boring in the earth, both for forming a bore hole and also for coring. They may be electric motors or fluid-operated motors. The motors are also useful in oil-field operations, such as tube cleaning, milling operations, cement drilling, and other operations where it is desired to rotate a rod at the end of which a tool is to be rotated. We refer to such motors as in-hole motors when designed to be run at the end of a pipe and adjacent to the operating tool, such as the drill bit. In the usual case, the rotor of the motor and the tool rotate with respect to a stator. When used in connection with a bore hole drill, it is connected to a conventional drill string composed, in the case of the drilling of well bores, of a drill pipe and drill collar as required. This string extends to the surface and is suspended from drilling lines. The tension in the lines is controlled, and the weight of the drill string on the drill bit is controlled. Where the in-hole motor is a hydraulic motor used as an in-hole motor in drilling, the liquid is the usual drilling fluid, i.e., mud or gas. It serves its usual function in the drilling operation, returning to the surface carrying the detritus, i.e., cuttings resulting from the drilling operation. However, in this combination, the circulating mud has an additional function; and that is to supply the hydraulic power to operate the hydraulic motor.
One of the primary problems resides in the design of the bearing system which will permit operations for periods of economic length.
In hydraulic motors, such as those referred to in my U.S. Pat. No. 3,857,655, issued Dec. 31, 1974, as is the case in all usual drilling operations, the load on the drill bit when the drill is on bottom is controlled by the tension imposed on the drilling lines. This tension is a lifting force on the drill string which subtracts from the dead weight of the drill string. The net weight is imposed on the bit.
The resistance to rotation of the bit, resulting from friction against the wall of the bore and the drilling effort, depends on the magnitude of the weight imposed on the bit. The torque on the bit required to overcome this friction and provide the drilling effort depends, in the case of hydraulic motors, on the pressure drop across the system.
This pressure drop is that required to move the drilling fluid through the drill pipe, drill collars, hydraulic motor, and the bit nozzles. In operating the system, it is desirable to maintain a constant torque. In the usual operation, the magnitude of the torque is determined by the difference of the input pressure at the surface when the bit is off bottom and the pressure when the bit is drilling ahead. The operator in the usual procedure controls the tension on the drilling line to maintain a substantially constant pressure and consequent torque.
This control imposes variable thrust loads on the bit and the thrust bearings.
In the in-hole motors described in the above patent, the weight on the bit is imposed by the drill string on the stator and through spring-loaded bearings to the shaft connected to the drill.
When the drill is off bottom, the weight of the rotor and connecting rod, shaft, drill bit, and hydraulic thrust is imposed on the housing. The weight of the drill string is off the shaft and drill bit.
The spring loading of the bearings as has been described in a copending application Ser. No. 354,954 filed Apr. 27, 1973, now U.S. Pat. No. 3,894,818 has the function of providing an initial precompressing load which prevents the separation of the races when the washing load is not imposed and thus avoids the displacement of the roller or ball bearings. It also reduces the hammering effect of vibrations and longitudinal vibration imposed to which the drill bit is exposed in drilling.
In normal drilling operations, the loading in the thrust bearing is controlled to give the required rate of penetration of the bit. This is accomplished by the driller who controls the tension in the drilling lines. The weight on the bit is a residual portion of the dead weight load.
It is frequently found that loads imposed on the thrust bearings are considerably in excess of the drilling load. These include the usual vibratory forces occurring during the rotation of the bit when roller bits are used, as the bit rides off the teeth. This phenomenon is well known.
These and other vibratory forces usually encountered in drilling impose serious oscillatory forces upon the system. Occasionally, the load rises to a substantial degree as the bit drills through a softer formation into a hard formation.
The drilling rate reduces, and the driller will slacken off on his line to impose more weight to maintain his drilling rate.
When spring-loaded bearings are used, the imposition of such excess loads may load the springs beyond their safe limit.
It is an object of our invention to design a spring-loaded bearing system which may take such overloads without stressing any of the springs beyond their safe limit.
In such case, at the usual lower operating load, the stiff springs sufficient to take such loads may be too stiff to operate as a suitable shock absorber.
It is an object of our invention to employ a spring-loaded bearing system whereby at the lower operating load the bearings are spring loaded by a spring system of moderate stiffness so as to permit the spring to dampen longitudinal vibrating forces imposed on the bearings. The bearing system is transformed into a system of greater stiffness when the load exceeds the predetermined upper limit. The stiffness of the springs is chosen for each so that at the maximum loading to which they are to be exposed they are not unduly stressed. We accomplish this effect by designing the spring system so that the deflection of springs at the applied loads does not exceed a predetermined fraction of the maximum deflection which the spring system can encounter.
The spring-loaded bearing system of our invention employs a plurality of spring systems arranged in series. The springs are of different stiffness and are arranged so that on application of a load on the plural spring systems the total deflection up to a maximum load on the spring system is unequally divided between the series springs. The total deflection is the sum of the unequal deflections of the spring system of lesser stiffness and the deflections of the spring system of greater stiffness.
In order to limit the deflection of the springs so that they are not overly stressed, we provide a stop for the spring system of lesser stiffness to terminate the deflection of the spring of lesser stiffness at a value less than the total deflection of the entire plural series systems under the maximum load contemplated. The deflection of the second spring system of greater stiffness is also less than the deflection of the entire plural series system at the maximum load contemplated. A stop is provided to permit the stiffer spring to deflect an amount beyond that at which the deflection of the spring system of lesser stiffness is stopped.
A further improvement is attained by the use of an additional spring system so arranged that, when the third spring is compressed by suitable means, a precompression load is imposed on the spring system described above. When the bit weight is applied to the plural spring systems, the precompression load is reduced. The plural spring system is partially unloaded, reducing the deflection of the plural spring and the deflection of the system.
The precompression load is not removed entirely, and the thrust bearings are under sufficient spring load so that contact of the roller or ball bearings and the races is maintained.
The system in our preferred embodiment is employed in in-hole motors used in oil well operations (see said U.S. Pat. No. 3,857,655). The system in our preferred embodiment is composed of a first and a second spring system to transmit the load of the housing onto the shaft and drill bit when the drill bit is on bottom; and a third spring system transmits the load of the rotor, connecting rod, shaft assembly and the hydraulic thrust load when the drill bit is off bottom. The third spring system is employed to impose a precompression load on the first and second spring systems.